Improvement of the safety of occupants in vehicles such as automobiles has been required in recent years, and the installation ratio of air bags has been improved. An air bag is a module wherein a sensor detects the impact a vehicle encounters during a head-on collision, a rear-end collision or a side collision of the vehicle, a gas is then blown into an expandable bag portion from an inflator to rapidly deploy and expand the air bag, and the cushioning of the air bag protects the occupant.
Conventional air bags are often installed in the front portion of a driver's seat and an assistant driver's seat, respectively, and are often installed to protect the faces and the upper half parts of the bodies of occupants principally during a head-on collision. A curtain-like air bag capable of corresponding to a side collision and a sideway rolling (hereinafter referred to as rollover) has recently been developed.
The curtain-like air bag is stored in, for example, a region from the front pillar side to the rear pillar side along the roof rail of the sidewall within an automobile, and designed to be expanded and deployed along the side windows during a collision. Moreover, for the curtain-like air bag, an expandable bag-like portion is formed at each of the plurality of sites, and the air bag is long and large and has a complicated shape. The curtain-like air bag is required to be excellent in storability (compactness). Furthermore, because the distance between the head portion of an occupant and the side glass window is short, the air bag must instantaneously enter between the occupant's head portion and the window glass to protect the head portion during a side collision. That is, the air bag is required to have a rapid deployment speed. Furthermore, correspondence of the air bag to a rollover accident in which the automobile side rolls several times is taken into consideration, and it is required that the internal pressure of the air bag not lower too much for a certain period. That is, the air bag is required to retain the internal pressure of 40 kPa or more for about 8 sec after expansion and deployment.
Furthermore, the internal pressure retention performance of the air bag is originally expected to function even after the air bag is exposed to a variety of environments. However, the air bag has actually not displayed the internal pressure retaining function under severe environmental test conditions such as heat aging, wet heat aging and cooling-heating cycle aging.
When the coating film thickness is increased in order to enhance the internal pressure retention performance, the air bag weight is undesirably increased. Moreover, the air bag cannot pass a severe environment test. In order to further increase the deployment speed, the woven fabric forming the base fabric must be made light weight.
There is one type of curtain-like air bag that is prepared by cutting a plurality of cloths, and sewing the cut cloths. In order to prevent air leakage caused by a sewing needle, the seams of the air bag are filled with a sealing agent, and the sites are sewn with a thick sewing thread for the purpose of ensuring burst resistance. As a result, the air bag has the following problems: the air bag has poor storability; and sewing the air bag takes a lot of time.
Japanese Unexamined Patent Publication (Kokai) No. 3-16852 discloses preparation of a bag-like high density woven fabric in which the periphery of the hollow weave portion is closed with a single-layer portion, and use of it as an air bag. However, such a woven fabric is formed out of a synthetic warp yarn and a synthetic weft yarn each having a total size of 500 dtex or more. It has a large amount of coating for the purpose of preventing air leakage, and a fabric weight of 700 g/m2. The air bag does therefore not sufficiently satisfy the requirement of light weight.
Japanese Unexamined Patent Publication (Kokai) No. 11-1876 discloses a fiber woven fabric having two coating layers. The first layer is formed by coating with a high elongation silicone, and the second layer is formed by coating with a high tear strength silicone. When the base fabric is coated with a high elongation silicone as the first layer, the air bag before aging shows good internal pressure retention. However, the high elongation silicone has poor adhesion to the woven fabric and the following problems result: the internal pressure retention after aging deteriorates after wet heat aging and cooling-heating cycle aging; and delamination of the silicone film sometimes takes place.
Japanese Patent Publication No. 2003-526557 discloses a technology of coating with a mixture of a silicone compound and a non-silicone compound as a first layer and a silicone compound as a second layer. The patent publication describes that the coating amount can be made small according to the technology, and that the internal pressure retention of the air bag can be maintained even after heat aging and wet heat aging. However, in conventional air bags, the adhesive strength of the first coating layer significantly lowers in the cooling region during cooling-heating cycle aging, and the air bag cannot withstand the initial pressure during expansion and deployment; as a result, the air bag cannot maintain the internal pressure retaining function.
Japanese Unexamined Patent Publication (Kokai) No. 2003-327910 discloses a silicone composition the silicone coating layer of which is not delaminated during deployment of an air bag and is excellent in internal pressure retention. However, there is no disclosure in the above patent specification regarding internal pressure retention during air bag deployment after aging.